public class LeetCode_5 {

    public static void main(String[] args) {
        LeetCode_5 p = new LeetCode_5();
        System.out.println(p.longestPalindrome("babad"));
    }

    public String longestPalindrome(String s){
        if (s == null || s.length() < 1) return "";
        int start = 0, end = 0;
        for (int i = 0; i < s.length(); i++){
            int len1 = expandAroundCenter(s, i, i);
            int len2 = expandAroundCenter(s, i, i+1);
            int len = Math.max(len1, len2);
            if (len > end - start){
                start = i - (len - 1) / 2;
                end = i + len / 2;
            }
        }
        return s.substring(start, end+1);
    }

    private int expandAroundCenter(String s, int left, int right){
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)){
            left--;
            right++;
        }
        return right - left - 1;
    }
}
